In typical digital data communication applications, a number of end user stations communicate among themselves or with a host computer using a high quality digital communication line. One example is that of the widely used Digital Data System (DDS), a fully synchronous digital communication system that offers high quality data service at a number of standard end user subrates. To communicate over the DDS network, an end user station must be coupled to the network by interfacing and data processing equipment. For example, in the Digital Cross-connect System (DCS) of the prior art shown in FIG. 1, each end user station 2000 is coupled to a channel unit 2002. The channel unit is in turn coupled to a DSO dataport 2004 by a DSX-O wire connection 2006. The outputs of the DSO dataports corresponding to all of the end user stations are then multiplexed together onto a single high-rate T1-line 2008 which is capable of accommodating as many as 24 DSO dataports. This T1 line is coupled to data processing circuitry 2010 comprising two DS1 line cards 2012 and 2016 and an application-specific hardware and software card 2014. Specifically, a first DS1 line card 2012 couples the T1 line to the hardware/software card, and a second DS1 line card 2016 couples the hardware/software card to the DDS network.
Several drawbacks characterize the DCS. Equipment and software instructions contained in the hardware/software card depend on the application in which the end user stations are operating.
One well-known example is the Multipoint Junction Unit (MJU) application. In this application, a number of branch stations (end users) share a DDS communication channel to a host station by using a DCS connection equipped with a hardware/software card dedicated to perform the MJU function. While the host may broadcast to all branch stations in the downstream direction, only one branch station can communicate with the host at any given time in the upstream direction. It is the individual end user stations that are responsible for exercising the proper line discipline, so that no two stations are transmitting simultaneously.
Another standard application consists of Subrate Data Multiplexing (SRDM). Here, the low rate data corresponding to a number of and user stations is multiplexed onto a single high rate DDS channel, to achieve increased transport efficiency. The multiplexing function is performed by a dedicated SRDM hardware/software card.
Hence, since a DCS implementation is equipped with hardware and software dedicated to a particular application, entirely different data processing equipment must be used from application to application. This drawback is particularly acute since end user stations must participate in a number of different applications.
Another problem with the DCS implementation arises from the fact that a single central data processor is responsible for processing data for all of the end user stations. Consequently, if this data processor malfunctions, all of the end user stations are affected.
Additionally, a substantial amount of wiring is needed to couple the various components comprising the DCS implementation, and these components are typically located at a number of different facilities. This results in difficulties in troubleshooting and maintaining the system.
Finally, the DCS implementation is expensive. A substantial expense is inherent in the amount of wiring that is needed, as mentioned above. Also, the incremental cost per end user branch is on the order of $500.